1. Field of the Invention
The present invention pertains to confocal optical imaging systems wherein a beam of light is passed through a plurality of optical elements and focused on a spot on the target with the reflected beam from the spot being reflected back through the optical elements to a detector, and more particularly, it pertains to such optical imaging systems wherein the transmitted and returned beam are passed through optical elements such as pinhole plates which can provide spurious reflections of the transmitted beam leading to unacceptable noise levels in the output signal.
2. Description of the Prior Act
Recently there has been some considerable interest in optical microscope systems for rapidly scanning specimens, such as semiconductor wafers or the like, at high degrees of resolution. For example, a coherent collimated beam from a laser can be focused on a very tiny spot on the specimen and the reflection from such spot directed back through the optical system to a detector wherein the reflectance can be utilized to inspect or measure various surface features of the specimen such as surface irregularities, profile, line widths, etc. In such a confocal optical system, a pinhole plate and various lenses are used which provide at least partially reflective surfaces which can create spurious signals or optical noise interferring with the reflected or returned beam from the spot on the specimen. Thus, maintaining an adequate signal-to-noise ratio in optical microscope systems of the foregoing type has been a substantial problem.
One general method of dealing with optical noise in a confocal optical system is shown in U.S. Pat. No. 4,139.263 to Lehureau et. al. In this patent there is disclosed an optical system wherein a nonpolarized laser beam is directed through a plurality of optical elements including a beam splitter with such transmitted beam being focused onto a spot on the data carrying target. The return beam from the target passes back through the optical elements and is deflected at the beam splitter to a photodetector. A quarter wave plate is positioned in the path of the beams between the focusing lens and the remainder of the elements in the optical arrangement so as to decouple the incident or transmitted wave from the return or reflected wave thereby reducing the optical noise in the signal received by the photodetector.
Another example of the use of a quarter wave plate in an optical system to reduce optical noise is shown in U.S. Pat. No. 3,919,698 to Bricot et. al.